1. Field of the Invention
This invention relates to an image forming apparatus for use in a reproduction device, such as a copying machine, a printer, a plotter, or a facsimile machine.
2. Description of Related Art
A known image forming apparatus forms an image using a toner flow control means having plural openings (hereinafter referred to as "apertures"). In this image forming apparatus, a voltage is selectively applied to the toner flow control means in accordance with image data to control toner particles to selectively pass through the apertures to form an image on a supporter (image forming medium) with the toner particles that pass through the apertures of the toner flow control means. This type of image forming apparatus is disclosed in the specification of U.S. Pat. No. 3,689,935, for example.
This image forming apparatus includes an aperture electrode unit serving as the toner flow control means, a potential supply means, and a toner supply means and a positioning means. The aperture electrode unit includes an insulating flat plate, a reference electrode, plural control electrodes and plural apertures. The reference electrode is continuously formed on one side surface of the flat surface. The plural control electrodes are electrically insulated from one another and formed on the other surface of the flat plate. The apertures are formed in a row in correspondence with the respective control electrodes to penetrate through the insulating flat plate, the reference electrode and the control electrodes.
The voltage supply means selectively applies a potential across the control electrodes and the reference electrode of the aperture electrode unit on the basis of the image data. The toner supply means supplies charged toner particles to the lower side of the aperture electrode unit so that the flow of the toner particles passing through the apertures is modulated in accordance with the potential applied to the aperture electrode unit. The positioning means serves to feed and position the supporter in a particle-flowing path so as to be movable relatively to the aperture electrode unit.
In the conventional image forming apparatus as described above, the aperture electrode unit is designed so that the reference electrodes are disposed on one surface of the flat plate and the plural control electrodes are disposed on the other surface of the flat plate. An electric field for controlling the charged toner is formed between the control electrodes and the reference electrode. Accordingly, to control the charged toner supplied to the peripheral portion of the aperture electrode unit from the toner supply means, a strong electric field must be formed between the control electrodes and the reference electrode. Therefore, a voltage supply means, which is capable of applying a high voltage, is required to form a strong electric field between the control electrodes and the reference electrode. So, the total cost of the apparatus is increased.
To solve this problem, the applicant of this application has proposed an image forming apparatus equipped with an aperture electrode unit 200 as shown in FIG. 10. The aperture electrode unit 200 comprises an polyimide insulating sheet 202 of 25 .mu.m thickness, plural control electrodes provided independently of one another and plural apertures 206.
The control electrodes 204 of 1 .mu.m thickness are provided on one surface of the insulating sheet 202. Each of the control electrodes 204 comprises an operating portion 204A disposed to surround each aperture 206 and a wiring portion (non-operating portion) 204B disposed to extend from each aperture 206 to one end portion of the insulating sheet 202. The apertures 206 are provided in correspondence with the respective control electrodes 204 to penetrate through the control electrodes 204 and the insulating sheet 202. These apertures 206 are designed to be substantially 150 .mu.m in diameter and are formed in a row in a longitudinal direction of the insulating sheet 202. The recording density of the aperture electrode unit 200 is set to 200 dpi(dot/inch).
The aperture electrode unit 200 is slightly pressed against a toner carry roller (not shown) to be in slight contact with the toner carry roller, and a voltage is applied across the control electrodes 204 and the toner carry roller. When the aperture electrode unit 200 thus constructed is applied to an image forming apparatus, an electric field is formed between the control electrodes 204 and the toner carry roller carrying charged toner thereon when a control voltage is applied to the control electrodes 204. So, a toner flow occurs between the control electrodes 204 and the toner carry roller. Therefore, as compared to the image forming apparatus described above, the toner flow can be controlled with an extremely lower voltage. In this case, in the vicinity of the contact portion between the aperture electrode unit 200 and the toner carry roller, the toner on the toner carry roller can pass through the apertures with the assistance of the electric field formed through the insulating sheet 202 between the operating portions 204A of the control electrodes 204 and the toner carry roller. The wiring portions 204B of the control electrodes are disposed at upstream and downstream sides in a rotation direction of the toner carry roller, that is, at upstream and downstream sides in a toner feeding direction.
However, the image forming apparatus as described above has the following problem. That is, controllability of charged toner particles in the aperture electrode unit is low. Thus, there occurs a phenomenon that toner is attached to even a non-image forming portion of a supporter (image forming medium), so that contrast in density of an image is degraded. By analyzing the cause of this phenomenon, it has been found out that conductive wires used to apply a control voltage to the control electrodes and drawn out from the control electrodes may have an adverse effect on an image forming process. That is, it is presumed that the following mechanism occurs to induce the above phenomenon. During an image forming process, when a control voltage is applied to the control electrodes through conductive wire portions drawn out to the upstream side of the toner feeding direction, toner particles on the toner carry roller are adsorbed onto a surface of the aperture electrode unit that faces the toner carry roller by an electric field formed by the conductive wire portions. These adsorbed toner particles are released from the surface of the aperture electrode unit when a non-image portion is formed on the supporter. At this time, some of these toner particles are blown out from the apertures to the supporter against an effect of the control voltage. So, the toner is unintentionally attached to the non-image forming portion on the supporter.
To solve this problem, it may be proposed to merely locate the conductive wiring portions of the control electrodes only at the downstream side of the toner feeding direction. However, this is practically very difficult for the following reason. For example, consider a case where an aperture electrode unit 210 as shown in FIG. 11A is used to obtain the same recording density (200 dpi) as the aperture electrode unit 200. FIG. 11B is an enlarged perspective view of a part of the aperture electrode unit 210. In this aperture electrode unit 210, the aperture pitch between respective apertures 216 is set to 125 .mu.m in order to attain the recording density of 200 dpi. The apertures 216 are arranged in a staggered form and designed to be substantially 150 .mu.m in diameter. Further, a control electrode 214 having a line width of about 30 .mu.m is provided to surround each of the apertures 215. The control electrode 214 for each aperture 216 is provided with a conductive wiring portion 218 having a line width of about 30 .mu.m. The line width of the control electrodes 214 and the conductive wiring portion 218 is more effective when as small as possible. In addition, although it is possible to set the line width to a value smaller than substantially 30 .mu.m, it is difficult in manufacturing process capability and manufacturing cost to narrow the line width below 30 .mu.m. Therefore, the line width of the control electrodes 214 and the conductive wiring portions is set to substantially 30 .mu.m. If the apertures 216, the control electrodes 214 and the conductive wiring portions 218 are designed as described above, the control electrodes 214 and the conductive wiring portions 218 adjacent to the control electrodes 214 are spaced from each other only at the minimum distance of 5 .mu.m. There is a problem that such a short distance is liable to induce a discharge between the control electrodes 214 and the adjacent conductive wiring portions 218. The line width of the control electrodes 214 and the adjacent conductive wiring portions 218 are set to be small, and the distance between the control electrodes 214 and the adjacent conductive wiring portions 218 are set be large to prevent occurrence of the discharge in this structure. However, as described above, it is difficult in manufacturing process capability and manufacturing cost to further narrow the line width of the control electrodes and the conductive wiring portions.